It is known to push the floating sludge together between floating sludge drain and floating sludge scraper and to evacuate it by lowering the intake edge of the drain. In this manner, the floating sludge cannot evade evacuation. By lowering the intake edge, an evacuating pressure is created causing the quick and complete evacuation of the floating sludge.
It has been found that this evacuating method with a stationary drain located at the end of the tank, which is used in longitudinal-type tanks, still is not the optimum solution. The long evacuating path allows that floating sludge evades the evacuating process underneath the scraper.